Acute thrombotic events including myocardial infarction and stroke have their peak incidence in morning hours. It has been postulated that among other factors, rising from the supine position after the overnight fast may play a role in the peak incidence of thrombotic events, possibly mediated by the rise in blood viscosity. This study was performed to examine the diurnal postural change in blood viscosity and whether oral hydration would affect this. Seventeen subjects were enrolled (7 male, 10 female; 14 white, 3 black), of these, 6 were excluded because of incomplete data. The remaining 11 subjects were analyzed for response to hydration. There were 6 females, 5 males with an average age of 72.7 +/- years. Racial breakdown was 8 white and 3 black. Results include: 1) There was a significant increase in blood viscosity noted between reclining and standing positions. 2) There was no significant difference in overall AM viscosity bewteen hydration and nonhydration conditions (this would be the average of reclining pre-hydration, as well as standing post-hydration conditions), (F=0.00, p=0.9697). There was a significant inter-subject difference in viscosity response to hydration (F=12.84, p=0.0001). There was also a significant difference in viscosity by position (F=283.62, p=0.0000), as well as a relatively less significant variation in AM response to position (F=11.96, p=0.0002). 3) There is a highly significant increase in Hct upon standing 39.7 to 43.6%, F=155.46, p<0.0001; there were no significant differences in elevation of Hct by time of day or hydration. This suggests that crystalloid hydration at this level does not overcome postural effects. It is also interesting to note that there was still a significant PM differential. This indicates that a 30 minute period of bed rest in the afternoon is sufficient to make the PM look like the AM. 4) Platelet activation was higher in the morning than in the afternoon and higher after standing than while reclining. No significant change was observed following hydration. These findings indicate a significant diurnal variation is present in blood viscosity and that there is a marked increase in blood viscosity and platelet aggregability upon rising to the standing position particularly in the morning. Oral hydration with 7.5 cc/Kg of electrolyte solution (Gatorade) did not significantly influence the morning increase in blood viscosity and platelet aggregability.